This invention relates in general to the use of eddy currents for detecting discontinuities, and relates in particular to an automated system for detecting cracks or other discontinuities in the periphery of fastener holes and detecting noise conditions which may mask the detection of a crack.
The safe and efficient maintenance of aircraft is of vital concern to the air transporation industry and to the defense of the nation. Stress upon aircraft structural members produced by flying causes metal fatigue and often produces hairline cracks in such members which are difficult to detect and require costly repair, and which if uncorrected can result in disastrous failures and accidents.
The workpiece commonly involved in fastener hole inspection is an aircraft structural member which contains a number of holes drilled for fastening or mounting purposes. There is naturally a great interest in providing an efficient and effective method for inspecting these holes for structural defects and flaws which produces consistent results and minimizes human intervention, interpretation, and error. Commonly, when a metal part has been subjected to excessive stress, metal fatigue results in hairline cracks which emanate radially from the periphery of the hole. Moreover, even holes newly drilled in structural metal parts can have hairline cracks. These cracks are often extremely small and are not readily apparent to the unaided eye. Fastener holes in critical structural components such as airframe members must be 100% inspected for cracks, and the inspection task is often complicated by the inaccessibility of a hole for viewing and by the presence of dirt, grease, or noncritical surface imperfections which obscure the presence of a crack or give a false impression that a crack is present.
It has long been known that the introduction of a magnetic field of alternating magnitude in current-conducting materials induces eddy currents in these materials. These eddy currents are detectable by a probe which senses the magnetic field produced in turn by the flowing of the eddy currents. During World War II, techniques were devised whereby an eddy current was induced in the vicinity of a fastener hole, and the eddy current was detected by a probe and displayed on a meter which could be interpreted by a human operator. Variations in the conductivity, permeability, or physical characteristics of the material, including variations caused by cracks or similar physical anomalies in the material, cause changes in the induced eddy current. A skilled operator could often determine the existence of a crack in the workpiece by observing fluctuations of the induced eddy current. This approach is at best haphazard due to meter fluctuations caused by positioning the probe into the hole. Further problems are encountered when the hole contained imperfections such as dirt, an out of round condition, scratches or corrosion. These conditions produce "noise" in the eddy current signal which complicated interpretation by masking the existence of a crack or by providing a spurious indication of a crack. Often, the operator will misinterpret signals produced by imperfections insignificant to structural integrity, rather than a crack, so that parts which had no crack are needlessly rejected.
The prior art discloses that it is possible to make eddy current devices which can be compensated for the placement of the probe into the hole and for its removal from the hole. Devices with compensation circuits which correct for placement and removal of the probe have been in existence for a number of years. One such device is the Model ED-520 eddy current instrument manufactured by Magnaflux Corporation, Chicago, Ill. These prior art devices, however, are susceptible to noise in the signal due to an insufficiently cleaned hole, scratches, an out of round condition, etc.
Correction for elimination of noise has proven a difficult problem. A device for distinguishing flaw signals from various noise signals in eddy current instrumentation circuitry is disclosed in Downs, U.S. Pat. No. 4,193,028. This device purports to distinguish an eddy current signal produced by a crack from an eddy current signal caused by noise. This device is premised on the theory that an eddy current signal produced by crack is characterized by a waveform which exceeds a predetermined negative threshold voltage followed immediately by a predetermined positive threshold voltage. According to the system of the Downs patent, a noise signal will not give an indication that a crack exists because of the low probability that a noise signal will pass the predetermined negative voltage threshold followed within the requisite time by a voltage swing exceeding the predetermined positive threshold voltage.
A problem encountered in the device of Downs is that there are circumstances in which a noise signal falsely indicates it is a crack signal. In these circumstances, the noise signal will meet the criteria for constituting a crack signal because the voltage level exceeds the predetermined negative voltage followed within the requisite time by a voltage level exceeding the predetermined positive level. It is rare for this noise signal to repetitively give false indication that a crack exists; however, it is annoying, forces a repetition of the test, and creates a situation of uncertainty for the inspector who must exercise judgement based on possibly-spurious information.
A further problem encountered with the use of Downs' device is that it employs monostable multivibrators triggered by the output of a differentiator. These one shot multivibrators are susceptible to being triggered by spurious noise signals which approach but do not exceed the predetermined threshold. This susceptibility to false triggering may result in a diagnosis of cracks in the workpiece when in fact no cracks exist. The susceptibility to false triggering appears to be compounded in the Downs arrangement because the one shots are triggered by the output of a differentiator circuit.
It is also known in the art to use digital techniques to perform signal detection and measurement functions. Moreover, it has been possible to program general purpose digital computers to perform calculations and computations upon measurements obtained in this fashion. However, the prior art does not disclose either the use of digital circuitry and analysis or the use of computing devices to gather eddy current data for the purpose of performing crack signal analysis. Nor does the prior art disclose any apparatus designed to detect the repetition of crack signatures within predetermined time windows for the purpose of distinguishing crack signals from spurious and non-repetitive noise signals. The prior art also does not disclose the employment of digital signal analysis techniques to distinguish noise signals from crack signals by taking sufficient samples of data to ensure the statistical validity of crack data over noise data. Thus, prior art devices only detect the existence of one eddy current signal signature which may or may not constitute a valid crack signature in light of further inspection.